The field of the present invention is systems for dissipating energy from rotary equipment.
Radial inflow turbines used to recover energy from pressurized gases as they are expanded are commonly referred to as turboexpanders. Such devices are widely used in air separation, natural gas liquification, ethylene plants, hydrogen purification, methyl-tertiary-butyl-ether (MTBE) processes, helium liquification, and geothermal energy recovery by Rankin Cycle among other applications. Three recent patents dealing with particular aspects of such turboexpanders are U.S. Pat. Nos. 4,287,758, issued Sep. 8, 1981; 4,300,869, issued Nov. 17, 1981; and 4,789,300, issued Dec. 6, 1988, the disclosures of which are incorporated herein by reference.
A process fluid or gas is presented in pressurized form to the turboexpander where the gas is expanded, resulting in a gas of lower pressure. The expansion of the gas provides rotational energy to the turboexpander. This expander energy may then be restored to the system by powering generators, blowers, compressors, pumps and the like or may be used for other purposes.
When such devices are employed in situations to achieve opportunistic energy recovery, the supply of process gas may not necessarily create continuous and sufficient rotational energy in the turboexpander. One may not want to recover expander energy when the expander energy is insufficient to warrant its recovery, when the availability of expander energy is intermittent, or when such recovery is not required by the end user for some other reason. In this case where expander energy is not recovered, expander energy must still be consumed, expended or otherwise dissipated to prevent damage to the turboexpander. Thus, the turboexpander still requires the provision of a load on the device.
Several methods to dissipate expander energy have been developed. Air blowers and oil brakes are two of the most widely used mechanisms for energy dissipation. Air blowers serve as a load to dissipate unwanted expander energy by using expander energy to power a fan. Similarly, oil brakes waste expander energy by acting as inefficient oil pumps.
When turboexpanders are utilized in cryogenic processes (i.e. processes which run at very low temperatures), any oil mist or droplets which migrate into the expander may greatly disrupt the process because the oil droplets will freeze in heat exchangers and will clog gas flow passages. This problem presents a major hazard for cryogenic processes.
Migrating droplets of oil or other lubricants may originate from various sources such as oiled or lubricated bearings. Although seals, such as shaft seals, prevent much of the oil in bearings from migrating to the expander, some oil leakage is inevitable. As a result, process plants have welcomed magnetic bearings which are oil-free. However, neither the air blower nor the oil brake is the preferred load when an expander having magnetic bearings is used to expand a hazardous fluid such as natural gas or hydrogen gas. An air blower may leak air into the turboexpander which may interact with the hazardous process fluid to form an explosive or highly combustible mixture. The use of an oil brake, on the other hand, defeats the purpose of substituting magnetic, oil-free bearings for oiled bearings since the oil brake subjects the process to the risk of oil contamination. Also, oil droplets from the oil brake are incompatible with the magnetic bearings.